Superplasticity Ppt 414wm

Superplasticity  Superplasticity is defined as a state in which a solid crystalline material is deformed well beyond its typical breaking point, often exceeding 1000% during tension.  Achieved in some fine-grained metals and ceramics at ~ 0.5 Tm  Requirements for superplasticity:  fine grain size, typically 1-10 μm,  a fine dispersion of thermally stable particles that act to pin the grain boundaries and  maintain the fine grain structure at the high temperatures.  The materials must also have a high strain rate sensitivity (>0.5) which prevents localized deformation at a reduced cross-section (necking).  At the macroscopic scale, a superplastically deformed material experiences uniform deformation, rather than localized necking, preceding fracture.

Superplasticity

Superplastic tensile deformation in Pb–62% Sn eutectic alloy tested at 415 K and a strain rate of 1.33 × 10−4 s−1; total strain of 48.5. (From M. M. I. Ahmed and T. G. Langdon, Met. Trans. A, 8 (1977) 1832.)

Superplasticity

Superplastic Deformation: Processes Used 1. Thermoforming 2. Blow Forming 3. Vacuum Forming

Plastic Deformation and Superplasticity

(a) Schematic representation of plastic deformation in tension with formation and inhibition of necking. (b) Engineering-stress–engineering-strain curves.

Strain-Rate Dependence in Superplastic Region

Strain-rate dependence of (a) stress and (b) strain-rate sensitivity for Mg–Al eutectic alloy tested at 350 ◦C (grain size 10 μm). (After D. Lee, Acta. Met., 17 (1969) 1057.)

Tensile Fracture Strain in Superplastic region

Tensile fracture strain and stress as a function of strain rate for Zr–22% Al alloy with 2.5-μm grain size. (After F. A. Mohamed, M. M. I. Ahmed, and T. G. Langdon, Met. Trans. A, 8 (1977) 933.)

Effect of Strain Rate Sensitivity on Superplasticity for Different Alloys

Effect of strain-rate sensitivity m on maximum tensile elongation for different alloys (Fe, Mg, Pu, Pb–Sr, Ti, Zn, Zr based). (From D. M. R. Taplin, G. L. Dunlop, and T. G. Langdon, Ann. Rev. Mater. Sci., 9 (1979) 151.)

Cavitation in Superplasticity

Cavitation in superplasticity formed 7475-T6 aluminum alloy (ε = 3.5) at 475 ◦C and 5 × 10−4 s−1. (a) Atmospheric pressure. (b) Hydrostatic pressure P = 4 MPa.

Effect of Grain Size on Elongation Effect of grain size on elongation: (A) Initial configuration. (B) Large grains. (C) Fine grains (10 μm) (Reprinted with permission from N. E. Paton, C. H. Hamilton, J. Wert, and M. Mahoney, J. Metal, 34 (1981) No. 8, 21)

Failure strains increase with superimposed hydrostatic pressure (from 0 to 5.6 MPa)

Effect of Grain Size on Elongation